The present invention relates generally to segmented aperture mirrors, and more specifically to their fabrication by electroforming.
A segmented aperture mirror (SAM) comprises a plurality of tightly packed mirror segments with identical surface curvatures mounted to a substrate structure of different curvature. SAMs have principally been used to smooth, or integrate, the spatial intensity variations of a laser beam, but other uses are possible depending on incident beam characteristics and segment and substrate geometries. One useful SAM for beam smoothing uses an array of square convex mirror segments mounted to a concave substrate structure to form a convex segmented aperture (CSA) mirror. A collimated beam of light incident on a CSA mirror is separated into many beams by reflection from individual segments. Each segment directs its portion of the reflected beam to overlap at the substrate focal plane. In this way, intensity variations in different areas of the incident beam are reduced by superimposing them.
U.S. Pat. No. 4,195,913 to Dourte et al teaches the fundamental construction and use of a convex segmented aperture mirror as an optical integrator. Its construction technique is sufficiently general to be feasible for other SAM geometries. Unfortunately, Dourte et al's construction teaching, including its segment construction, segment attachment method, segment separation and suggested water cooling of the substrate to which the segments are attached, limits both the accuracy and the level of incident energy allowable for such a device. Convex segmented aperture mirrors are generally used in the optical trains of test systems for studying the effects of high energy laser irradiation, requiring protection against heat distortion and other thermal effects. Individually cooling each segment has been suggested as a means for improving the thermal performance of a discrete segment CSA mirror under such high energy irradiation. This approach suffers from the complexity of a multiplicity of coolant connections required to cool each segment and the adverse impact of the connections on segment mounting.
Many of the problems associated with discrete segment mirrors can be overcome by fabricating a seamless continuous faceplace, containing the desired SAM optical surface, by electroforming. Electroforming is a variation of electroplating involving the formation of a removable layer of metal which conforms exactly to the shape of the surface of a master. U.S. Pat. No. 3,428,533 to Pichel, U.S. Pat. No. 3,378,469 to Jochim, and U.S. Pat. No. 1,871,770 to Bart, for example, teach various uses of electroforming to make reflecting mirrors from a continuous master. While each of these patents, and the other electroforming prior art, provide valuable teachings adaptable to constructing a SAM, those teachings are insufficient to successfully construct an electroformed SAM able to withstand high energy laser irradiation.
For example, electroforming of optical surfaces requires control of bath and plating parameters to limit the springback of the electroform, when separated from the master, to acceptable levels. Electroformed optics are generally thin (less than 0.100") and freestanding when removed from the master. Adding cooling will result in a thin unsupported cooled faceplate with unacceptable distortions due both to springback and deformation during separation form the master and to induced cooling loads. Bonding a rigidizing substrate to the faceplate to minimize these distortions can create distortions of its own due to bond shrinkage or substrate stresses. Additionally, the properties of the bonding material must not reduce the stiffness and stability of the substrate. The bonded assembly must maintain its integrity and not disbond.
It is, therefore, a principal object of the present invention to provide an electroforming method for making a cooled CSA type SAM suitable for use with high energy laser irradiation.
It is another object of the present invention to provide an electroforming method for fabricating cooled SAMs of other geometries suitable for use with high energy laser irradiation.
It is a further object of the present invention to provide an improved method for fabricating discrete segment SAMs for use as electroforming masters or for use with low power laser irradiation.
It is yet another object of the present invention to provide an improved bonding method for rigidizing thin optical electroforms.
A feature of the present invention is that a very large SAM assembly may be fabricated by assembling onto a rigid structure many identical SAM tiles made from a small tile master.
Another feature of the present invention is that it allows fabricating many SAM tiles at once from multiple working masters made from a single master, thereby reducing the cost and complexity of fabricating a large SAM assembly.
An advantage of the present invention is that it provides an optical quality cooled surface without the prior art complexity, sealing problems and absorption losses of a conventional CSA type SAM with discrete, cooled segments.